Copending patent application Ser. No. 07/213,719 filed June 30, 1988 now U.S. Pat. No. 4,885,802 by Lawrence H. Ragan entitled Wristwatch Receiver Architecture describes an FM radio receiver suitable for paging applications. The radio receiver shown in the referenced patent application is constructed on a single integrated circuit chip with only a small number of external components. Among the reasons that it is desirable to minimize the number of off-chip components is that such components increase the cost of the unit, they occupy room in the package, and they increase the assembly cost.
The receiver shown in the above referenced patent application includes two off-chip band pass filters. It is possible to build band pass filters on an integrated circuit chip using gyrators to simulate inductors; however, at high frequencies such circuits tend to be unstable. That is, the pass band and the stop band of filters built on an integrated circuit chip using gyrators tend to vary over time because the simulated inductance of a gyrator is a function of the DC bias current through the circuit and it is very difficult to precisely control DC bias current in an integrated circuit chip. In an FM radio receiver, the IF filter is a critical component relative to insuring proper discrimination between stations, hence, it is essential that the IF filter have the proper frequency response, hence, prior art FM receivers generally use external IF filters.
The IF stage of FM radio receivers typically use two 10.7 megahertz two pole filters in series. The radio receiver shown in the above referenced patent application includes two 10.7 megahertz filters as off-chip components.
Adaptive filters, that is, filters whose frequency response is adjustable in response to external signals are known. For example, a paper by W. B. Mikhael and F. F. Yassa entitled "Stable High Order Continuous Adaptive Filters" IEE Int. Symp. on Circuits and Systems, pp 666-669, Rome, Italy, May 1982 describes a filter which is adapted in response to an RMS measurement of an error signal. It is also know that one can make an adaptive filter which automatically adapts so that the filter has the same characteristics as does a fixed reference filter. For example a paper by D. A. Johns, W. M. Snelgrove, and A. S. Sedra entitled "Continuous-Time Analog Adaptive Recursive Filters" published in ISCAS 1989 pages 667-669 describes how an adaptive filter and a reference filter can be connected s that the adaptive filter's frequency response is made to match that of the reference filter. According to the technique described in this paper, white noise is injected into both the adaptive filter and the reference filter. The adaptive filter is then adjusted so that it produces the same response to the white noise as does the reference filter. At this point, the transfer function of the adaptive filter matches the transfer function of the reference filter.
If an adaptive filter is built on an integrated circuit chip, the filter will only hold the frequency response to which it is adjusted for a relatively short period of time due to the fact that the DC bias current in the integrated circuit will drift thereby changing the value of the inductances in the circuit.